1. Field of the Invention
This invention relates to a power supply device and, more particularly, to a power supply device which can be used suitably for electro-photographic copiers, printers and so forth.
2. Related Background Art
Various image-forming apparatuses such as electro-photographic copiers and printers are well known in the art. These apparatuses require a high DC or AC or superimposed AC-DC voltage for a charging step, a developing step and so forth. Particularly, a high superimposed AC-DC voltage is provided as developing bias to a developing unit. A usual power supply for providing a developing bias comprises a low frequency AC output step-up transformer and a DC/DC inverter coupled to the transformer for superimposing a DC voltage on the AC output.
In the prior art power source of this kind, however, the low frequency AC output step-up transformer for generating an AC component of the developing bias is large in size. In addition, the frequency of the AC component of the bias is in the audible frequency range, and therefore considerable noise is generated. It is very difficult to reduce this noise. Further, the duty ratio of the transformer can not be varied greatly from 50% due to unbalanced excitation of the transformer.
It is proposed to modulate a primary side input to the DC/DC inverter according to a low frequency signal to thereby modulate a secondary side rectified output of the inverter, thus obtaining a high voltage, in which a modulation component is superimposed on a DC component (as disclosed in U.S. Pat. Nos. 4,731,720 and 4,727,463).
In this case, however, if a pulse wave is used as the low frequency signal, the response of the falling of the secondary side output to the falling of the pulse wave is dull, and it is impossible to obtain an adequate output.